<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        const str = 'div#demo.demo';

        let tag = '';
        let id = '';
        let classArr = [];
        
        for (var i = 0; i < str.length; i ++) {
            if (str[i] == '#') {
                break;
            }
            tag += str[i];
        }
        i++

        for (; i < str.length; i ++) {
            if (str[i] == '.') {
                break;
            }
            id += str[i];
        }

        classArr = str.slice(i + 1).split('.');

        const tagStr = `<${tag} id="${id}" class="${classArr.join(' ')}"></${tag}>`

        console.log(tagStr);
    </script>
</body>
</html>